1. Field of the Invention
The present invention relates to a signal distribution system for effectively distributing a high-speed digital signal on a bus transmission path to many reception circuits without degrading its better characteristic.
2. Description of the Related Art
Conventionally, if a digital signal is to be distributively supplied to digital reception circuits RC.sub.1, . . . , RC.sub.N from more than one digital transmission circuit DV (one is shown in FIG. 1), a signal distribution system may be employed which supplies such a signal to the associated circuits via a bus transmission path BL.sub.1 on which one terminal of transmission line S terminates in an output-determining potential E, as shown in FIG. 1, via a resistor R.sub.3 equal to a line-to-ground characteristic impedance Z.sub.0.
In a differential type signal distribution system, as shown in FIG. 2, a differential bus transmission path BL.sub.2 is provided on which at least each terminal of a pair of transmission lines S.sub.1, S.sub.2 terminates in a potential E via resistors R.sub.3 and R.sub.4 equal to a corresponding line-to-ground characteristic impedance Z.sub.0. A pair of differential output terminals of a digital transmission circuit DV and a pair of differential input terminals of each of N number of digital reception circuits RC.sub.1 to RC.sub.N are connected to the transmission lines S.sub.1 and S.sub.2, respectively. A differential digital signal is delivered from the digital transmission circuit DV onto the transmission lines S.sub.1 and S.sub.2 and distributively supplied to the N number of digital reception circuits RC.sub.1 to RC.sub.N over the differential bus transmission path BL.sub.2.
If the fundamental frequency of the digital signal to be distributed becomes high in the aforementioned signal distribution system, the input impedance Z.sub.L, particularly the parasitic capacitance C.sub.L ', of the digital reception circuits RC.sub.1 to RC.sub.N (only RC.sub.1 is shown in detail) as shown in FIG. 2 cannot be disregarded, causing a drop in the input impedance and hence a mismatching among respective branch points. This provides a cause for a multiple reflection echo and degrades a signal waveform being transmitted.
Conventionally, as shown in FIG. 3, the differential input terminals of respective digital reception circuits RC.sub.1 to RC.sub.N are connected respectively through branch resistors R.sub.1 ' and R.sub.2 ' to transmission lines S.sub.1 and S.sub.2 on a bus transmission path BL.sub.2. By doing so, an input impedance equivalent to that of the respective reception circuit is increased to reduce a mismatching at the branch point and hence to suppress a multiple reflection echo. In this case it is common practice to selectively set the branch resistors R.sub.1 ' and R.sub.2 ' to be about one half the characteristic impedance Z.sub.0 on the transmission path BL.sub.2. This means enables the retentivity of a signal waveform to be greatly improved over that in the absence of any branch resistors.
If the N number of the reception circuit exceeds a certain value even in the presence of such resistors, the input impedance Z.sub.L of the reception circuit cannot be disregarded with respect to the characteristic impedance Z.sub.0 on the transmission path BL.sub.2, providing a cause for a mismatching. If the mismatching occurs, the multiple reflection echo is increased, resulting in a degraded signal waveform. In order to avoid such an inconvenience, it is only necessary to increase the value of the branch resistor with an increase in the N number of the reception circuits. Even in this case, however, the cutoff frequency determined by an impedance with the branch resistance initially built in is lowered and it is not possible to transmit a high-speed digital signal. Thus there is a limit on the increase of the branch resistance.